This invention relates to a device for transferring digital data between two storage devices in a digital data processing system. The preferred embodiment is described in connection with a system for establishing and maintaining one or more duplicate or "shadow" copies of stored data to thereby improve the availability of the stored data.
A typical digital computer system includes one or more mass storage subsystems for storing data (which may include program instructions) to be processed. In typical mass storage subsystems, the data is actually stored on disks. Disks are divided into a plurality of tracks, at selected radial distances from the center, and sectors, defining particular angular regions across each track, with each track and set of one or more sectors comprising a block, in which data is stored.
Since stored data may be unintentionally corrupted or destroyed, systems have been developed that create multiple copies of stored data, usually on separate storage devices, so that if the data on one of the copies is damaged, it can be recovered from one or more of the remaining copies. Such multiple copies are known as a "shadow set." In a shadow set, typically data that is stored in particular blocks on one member of the shadow set is the same as data stored in corresponding blocks on the other members of the shadow set. It is usually desirable to permit multiple host processors to simultaneously access (i.e., in parallel) the shadow set for read and write type requests ("I/O" requests).
A new storage device or "new member" is occasionally added to the shadow set. For example, it may be desirable to increase the number of shadow set members to improve the data availability or it may be necessary to replace a shadow set member that was damaged. Because all shadow set members contain the same data, when adding a new member, all of the data stored on the active members is copied to the new member.